We explored the biology of IL-15 and showed that efficient production of IL-15 is possible only by co-expression in the same cell with the so-called IL-15 Receptor-alpha. We also showed that a second form of IL-15 (SSP IL-15) previously identified in humans and rodents as intracellular or nuclear IL-15 is also efficiently secreted from the cells when co-expressed with the IL-15 Receptor alpha. These results shed new light in the biology and regulation of IL-15 and provide methods for the efficient production and clinical application of this cytokine. Cell lines overproducing soluble bioactive IL-15/IL-15 Receptor alpha heterodimers have been constructed and were used for the production of the bioactive form of IL-15 found in the body and for the development of cGMP material for clinical trials. IL-15 is of interest due to its ability to stimulate the growth, activation and survival of lymphocytes, including CD8 and NK cells. Thus, IL-15 has been considered for cancer immunotherapy and for support of the growth of cytotoxic cell clones after adoptive transfer. Other proposed uses of IL-15 are in lymphopenia, in supporting NK cell growth and activation after NK transfer, and as vaccine adjuvant. We have shown that IL-15 injection accelerates the recovery of lymphocytes in mice rendered lymphopenic after treatment with cytotoxic drugs. We also showed that hetIL-15 can replace the need for lymphodepletion in Adoptive Cell Transfer (ACT), since the transferred cells can survive, proliferate and enter the tumors after hetIL-15 treatment. This may have important clinical implications for ACT protocols. We have used the previously developed technologies of RNA optimization to optimize expression of IL-15 cytokine, and have shown that we can over-produce bioactive cytokine after DNA delivery in mice and macaques. DNA delivery of vectors expressing heterodimeric IL-15 leads to systemically active levels of cytokine and the increased proliferation of NK and T cells. We have shown that hetIL-15 greatly increases lymphocyte infiltration in several tumors in mouse models and in macaques, suggesting a general method to increase lymphocyte infiltration, which is associated with anti-tumor activity. On the basis of these results, we have initiated first-in-human clinical trials of hetIL-15 in metastatic cancers and also in combination with anti-PD-1 check point inhibitor (NCT02452268; collaboration with Novartis). In addition to cancer immunotherapy, IL-15 has generated strong interest for clinical use to treat HIV infection, especially in protocols targeting viral eradication or a functional cure. The use of IL-15 as an immune therapeutic agent against HIV infection is based on its effects as a growth factor and key regulator of cytotoxic responses mediated by both the innate (NK cells) and the adaptive (CTL) arms of the immune system. Using hetIL-15 treatment in the RM model for HIV infection, we have demonstrated that: (i) a regimen with increasing dosing (step-dose) of hetIL-15 is well tolerated by different routes of delivery and increases T lymphocytes (CD8, CD4 and gamma/delta) and NK cells with high granzyme content in peripheral blood, mucosal sites and LN. (ii) Importantly, hetIL-15 treatment promotes the entrance of cytotoxic (GrzB+) CD8+ T cells in the B cell follicles, areas within the LN where CTL are typically excluded and where SIV/HIV infected follicular helper CD4+ T cells reside. hetIL-15 treatment led to significant decrease in cell-associated viral RNA within the LN as well as in plasma viremia in SHIV infected macaques. (iii) We also showed that hetIL-15 can enhance ADCC in LN, which provides an additional mechanism of elimination of infected cells. We have also used optimized vectors to express IL-12 cytokine in animals. Efficient expression results in bioactive levels, which increase immune response after DNA vaccination, thus becoming important molecular adjuvant for our vaccines. We have also developed methods for large-scale isolation and purification of exosomes. We have shown that hetIL-15 is incorporated in exosomes and have produced sufficient quantities for animal experiments. We propose that the versatility of exosomes can be used for the delivery of immunotherapy exosomes to disease sites (tumor or lymphoid tissue). We developed methods for the efficient delivery of exosomes to tumor sites.